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Explanation: Is that a cloud or an alien spaceship? It's an unusual and sometimes dangerous type of thunderstorm cloud called a supercell. Supercells may spawn damaging tornados, hail, downbursts of air, or drenching rain. Or they may just look impressive. A supercell harbors a mesocylone -- a rising column of air surrounded by drafts of falling air. Supercells could occur over many places on Earth but are particularly common in Tornado Alley of the USA. Pictured above are four time lapse sequences of a supercell rotating above and moving across Booker, Texas. Captured in the video are new clouds forming...

Explanation: What creates these long and nearly straight grooves on Mars? Dubbed linear gullies, they appear on the sides of some sandy slopes during Martian spring, have nearly constant width, extend for as long as two kilometers, and have raised banks along their sides. Unlike most water flows, they do not appear to have areas of dried debris at the downhill end. A leading hypothesis -- actually being tested here on Earth -- is that these linear gullies are caused by chunks of carbon dioxide ice (dry ice) breaking off and sliding down hills while sublimating into gas, eventually completely...

Explanation: The first APOD appeared eighteen years ago today, on 1995 June 16. Although garnering only 14 pageviews on that day, we are proud to estimate that APOD has now served over one billion space-related images over the past eighteen years. That early beginning, along with a nearly unchanging format, has allowed APOD to be a consistent and familiar site on a web frequently filled with change. Many people don't know, though, that APOD is now translated daily into many major languages. We again thank our readers, astrophotographers, and NASA for their continued support, but ask that any potentially congratulatory...

Explanation: Over a five hour period last Tuesday morning, exposures captured this tantalizing view of meteor streaks and the Milky Way in dark skies above Las Campanas Observatory in Chile. During that time, astronomers had hoped to see an outburst from the gamma Delphinid meteor shower as Earth swept through the dust trail left by an unknown comet. Named for the shower's radiant point in the constellation Delphinus, a brief but strong outburst was reported in bright, moonlit skies on June 10, 1930. While no strong Delphinid meteor activity was reported since, an outburst was tentatively predicted to occur again...

Explanation: Sharpless 115 stands just north and west of Deneb, the alpha star of Cygnus the Swan in planet Earth's skies. Noted in the 1959 catalog by astronomer Stewart Sharpless (as Sh2-115) the faint but lovely emission nebula lies along the edge one of the outer Milky Way's giant molecular clouds, about 7,500 light-years away. Shining with the light of ionized atoms of hydrogen, sulfur, and oxygen in this Hubble palette color composite image, the nebular glow is powered by hot stars in star cluster Berkeley 90. The cluster stars are likely only 100 million years old or so and...

(Image: NASA/JPL-Caltech/University of Arizona) A rolling stone gathers no moss – but on Mars it can nevertheless cloak itself in mystery. This NASA image shows the track of a boulder that rolled across the Nili Fossae region of Mars. For now it is anyone's guess what set the rock in motion. This false-colour picture (click on it for higher resolution) was posted on 7 June to the Beautiful Mars Tumblr feed, a collection of high-resolution shots from the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter. It shows dark, jagged tracks left in the soil by a lumpy boulder, probably...

Explanation: You can see four planets in this serene sunset image, created from a series of stacked digital exposures captured near dusk on May 25. The composite picture follows the trail of three of them, Jupiter, Venus, and Mercury (left to right) dropping toward the western horizon, gathered close in last month's remarkable triple planetary conjunction. Similar in brightness to planet Mercury, the star Elnath (Beta Tauri) is also tracked across the scene, leaving its dotted trail still farther to the right. Of course, in the foreground are the still, shallow waters of Alikes salt lake, reflecting the striking colors...

Explanation: For the first time, the entire surface of planet Mercury has been mapped. Detailed observations of the innermost planet's surprising crust have been ongoing since the robotic MESSENGER spacecraft first passed Mercury in 2008 and began orbiting in 2011. Previously, much of the Mercury's surface was unknown as it is too far for Earth-bound telescopes to see clearly, while the Mariner 10 flybys in the 1970s observed only about half. The above video is a compilation of thousands of images of Mercury rendered in exaggerated colors to better contrast different surface features. Visible on the rotating world are rays...

JPL video here

Explanation: What's happening in the NGC 3582 nebula? Bright stars and interesting molecules are forming. The complex nebula resides in the star forming region called RCW 57. Visible in this image are dense knots of dark interstellar dust, bright stars that have formed in the past few million years, fields of glowing hydrogen gas ionized by these stars, and great loops of gas expelled by dying stars. A detailed study of NGC 3582, also known as NGC 3584 and NGC 3576, uncovered at least 33 massive stars in the end stages of formation, and the clear presence of the complex...

Explanation: Where are the hottest stars in the nearest galaxies? To help find out, NASA commissioned its Earth-orbiting Swift satellite to compile a multi-image mosaic of the neighboring Large Magellanic Cloud (LMC) galaxy in ultraviolet light. The above image shows where recently formed stars occur in the LMC, as the most massive of these young stars shine brightly in blue and ultraviolet. In contrast, visible in an image roll-over, a more familiar view of the LMC in visible light better highlights older stars. On the upper left is one of the largest star forming regions known in the entire Local...

Credit: Video courtesy of Cooper Downs Deep inside the sun's atmosphere, temperatures reach millions of degrees—so hot that even the best-shielded spacecraft can't go there (even at night). But natural objects that pass exceptionally close to the sun do provide scientists opportunities to directly probe the solar corona. Enter sun-grazing comets, such as comet Lovejoy, which whizzed within 140,000 kilometers of the sun's surface in mid-December 2011 (as seen in the first 20 seconds of the video). When a comet is far from the sun, its tail acts like a weather vane in the solar wind (the torrent of...

Super-high-energy galactic gamma rays have trillions of times more energy than visible light, and they disappear in the atmosphere before they hit the Earth’s surface. So if you want to detect these mysterious phenomena, a regular telescope isn’t much help. To learn about the highest-energy gamma rays, scientists build elaborate observatories, and one of the most advanced is the new High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, high in the mountains about six hours from Mexico City. When it’s completed, it will be the only facility in the world that can detect the highest-energy gamma rays, with energies of up to...

Explanation: Have you ever seen an aurora? Auroras are occurring again with increasing frequency. With the Sun peaking at its eleven year maximum in aurora-triggering activity, it is exhibiting a greater abundance of sunspots, flares, and coronal mass ejections. Solar activity like this typically expels charged particles into the Solar System, some of which impact Earth's magnetosphere and trigger Earthly auroras. In late 2010, the above timelapse displays of picturesque auroras were captured above Tromso, Norway. Curtains of auroral light, usually green, flow, shimmer and dance as energetic particles fall toward the Earth and excite atoms of air high up...

Explanation: Many bright nebulae and star clusters in planet Earth's sky are associated with the name of astronomer Charles Messier, from his famous 18th century catalog. His name is also given to these two large and remarkable craters on the Moon. Standouts in the dark, smooth lunar Sea of Fertility or Mare Fecunditatis, Messier (left) and Messier A have dimensions of 15 by 8 and 16 by 11 kilometers respectively. Their elongated shapes are explained by an extremely shallow-angle trajectory followed by the impactor, moving left to right, that gouged out the craters. The shallow impact also resulted in two...

Explanation: The bright clusters and nebulae of planet Earth's night sky are often named for flowers or insects. Though its wingspan covers over 3 light-years, NGC 6302 is no exception. With an estimated surface temperature of about 250,000 degrees C, the dying central star of this particular planetary nebula has become exceptionally hot, shining brightly in ultraviolet light but hidden from direct view by a dense torus of dust. This sharp and colorful close-up of the dying star's nebula was recorded in 2009 by the Hubble Space Telescope's Wide Field Camera 3, installed during the final shuttle servicing mission. Cutting...

Explanation: How big is our Sun compared to other stars? In a dramatic and popular video featured on YouTube, the relative sizes of planets and stars are shown from smallest to largest. The above video starts with Earth's Moon and progresses through increasingly larger planets in our Solar System. Next, the Sun is shown along as compared to many of the brighter stars in our neighborhood of the Milky Way Galaxy. Finally, some of the largest stars known spin into view. Note that the true sizes of most stars outside of the Sun and Betelgeuse are not known by direct...

Explanation: Except for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial band. Its classic appearance is understood to be due to our own perspective, though. The recent mapping of the expanding nebula's 3-D structure, based in part on this clear Hubble image, indicates that the nebula is a relatively dense, donut-like ring wrapped around the middle of a football-shaped cloud of glowing gas. The view from planet Earth looks down the long axis of the football, face-on to the ring. Of course, in this well-studied example of a planetary nebula, the glowing material does...

Explanation: Few astronomical sights excite the imagination like the nearby stellar nursery known as the Orion Nebula. The Nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud. Many of the filamentary structures visible in the above image are actually shock waves - fronts where fast moving material encounters slow moving gas. The Orion Nebula spans about 40 light years and is located about 1500 light years away in the same spiral arm of our Galaxy as the Sun. The Great Nebula in Orion can be found with the unaided eye just below and...

Explanation: Could life ever have existed on Mars? To help find out, humanity landed the Curiosity rover on Mars last August. To make sure the car-sized explorer survived the interplanetary trip and dramatic landing intact, the above image and others was taken peering at, under, and around Curiosity. Pictured above in this unusual vista are three of Curiosity's six wheels, each measuring about half a meter across. In recent months, Curiosity has been exploring the surroundings of an area dubbed Yellowknife Bay. Analyses of data taken by Curiosity's cameras and onboard laboratories has provided strong new evidence that Mars could...

Explanation: What kind of cloud is this? A roll cloud. These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form. Roll clouds may actually have air circulating along the long horizontal axis of the cloud. A roll cloud is not thought to be able to morph into a tornado. Unlike a similar shelf cloud, a roll cloud, a type of Arcus...

Explanation: Have you ever hiked the Queen's Garden trail in Bryce Canyon, Utah, USA, planet Earth? Walking along that path in this dark night skyscape, you can almost imagine your journey continues along the pale, luminous Milky Way. Of course, the name for our galaxy, the Milky Way (in Latin, Via Lactea), does refer to its appearance as a milky band or path in the sky. In fact, the word galaxy itself derives from the Greek for milk. Visible on moonless nights from dark sky areas, though not so bright or quite so colorful as in this image, the glowing...

On galaxies and bosons, stars and quarks. In physics, truth does not always equal beauty. The Milky Way: An Insider’s Guide By William H. Waller (Princeton University Press, 296 pages, $29.95) A Palette of Particles By Jeremy Bernstein (Belknap Press of Harvard University, 224 pages, $18.95) THE BRITISH PHILOSOPHER J.L. Austin coined the handy phrase “medium-sized dry goods” to describe the world of everyday phenomena that the human nervous system is best suited to cope with, phenomena ranging in size from a grain of dust to a landscape. Within that range our senses and cognition are at home. All our...

Explanation: The Eagle Nebula and the Swan Nebula span this broad starscape, a telescopic view of the Sagittarius spiral arm toward the center of our Milky Way galaxy. The Eagle, also known as M16, is left, above center, and the Swan, or M17 at the lower right. The deep, wide-field image shows the cosmic clouds as brighter regions of active star-formation. They lie along the spiral arm suffused with reddish emission charactistic of atomic hydrogen gas, and dusty dark nebulae. In fact, the center of both nebulae are locations of well-known close-up images of star formation from the Hubble Space...

Explanation: While most spiral galaxies, including our own Milky Way, have two or more spiral arms, NGC 4725 has only one. In this sharp color composite image, the solo spira mirabilis seems to wind from a prominent ring of bluish, newborn star clusters and red tinted star forming regions. The odd galaxy also sports obscuring dust lanes a yellowish central bar structure composed of an older population of stars. NGC 4725 is over 100 thousand light-years across and lies 41 million light-years away in the well-groomed constellation Coma Berenices. Computer simulations of the formation of single spiral arms suggest that...

Explanation: Ten thousand years ago, before the dawn of recorded human history, a new light would have suddenly have appeared in the night sky and faded after a few weeks. Today we know this light was from a supernova, or exploding star, and record the expanding debris cloud as the Veil Nebula, a supernova remnant. This sharp telescopic view is centered on a western segment of the Veil Nebula cataloged as NGC 6960 but less formally known as the Witch's Broom Nebula. Blasted out in the cataclysmic explosion, the interstellar shock wave plows through space sweeping up and exciting interstellar...

Explanation: The 16th century Portuguese navigator Ferdinand Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two fuzzy cloud-like objects easily visible to southern hemisphere skygazers are known as the Clouds of Magellan, now understood to be satellite galaxies of our much larger, spiral Milky Way galaxy. About 160,000 light-years distant in the constellation Dorado, the Large Magellanic Cloud (LMC) is seen here in a remarkably deep, colorful, and annotated composite image. Spanning about 15,000 light-years or so, it is the most massive of the Milky...

Explanation: Have you ever seen a little rainbow off to the side of the Sun? Rare but rewarding to see, such spectacles are known as sundogs, mock suns or parhelia. Sundogs are just sunlight refracting through hexagonal falling ice crystals in the Earth's atmosphere. When thin ice crystals flitter down nearly horizontally, they best refract sunlight sideways and create sundogs. Alternatively, randomly oriented ice crystals may create a complete circular sun halo. Sundogs occur 22 degrees to each side of a setting or rising Sun, although sometimes nearby clouds can block one or both. The above image was taken through...

Explanation: As planet Earth approached the plane of the Comet PanSTARRS (C/2011 L4) orbit on May 23rd, comet watchers were treated to this view of its magnificent anti-tail. The long, narrow anti-tail stretches to the right across this frame for nearly 4 degrees or about 8 times the angular size of the full Moon. The tail trails along the comet's orbit as it leaves the inner solar system behind. An almost edge-on perspective from near the outbound comet's orbital plane enhances the view of the anti-tail and makes it seem to point in the sunward direction, only apparently contrary to...

Explanation: This bat-like apparition does not shine on clouds passing over Gotham city. Instead, the cloud bank in silhouette against a colorful lunar corona was spotted on the evening of May 18 over Cochem, Germany from the banks of the river Mossele. The lunar corona is formed as bright moonlight is diffracted by water droplets in thin clouds drifting in front of the lunar disk. Below it lies the region's historic Cochem Castle dating from the 11th century, and not Wayne Manor. Still, regardless of your location on planet Earth it is well worth scanning the evening skies this weekend,...

It's 1.7 miles long. Its surface is covered in a sticky black substance similar to the gunk at the bottom of a barbecue. If it impacted Earth it would probably result in global extinction. Good thing it is just making a flyby. Asteroid 1998 QE2 will make its closest pass to Earth on May 31 at 1:59 p.m. PDT. Scientists are not sure where this unusually large space rock, which was discovered 15 years ago, originated. But the mysterious sooty substance on its surface could indicate it may be a result of a comet that flew too close to the...

Explanation: A close series of consecutive exposures are combined in this intriguing composite of the Full Moon slowly crawling, across the sky. Beginning on the upper right at 19:42 UT and ending at 22:14 UT on April 25, the sequence follows the Moon from Germany as it passes through Earth's shadow in a partial lunar eclipse. Near the top, the Moon just grazes the southern edge of Earth's dark central shadow, or umbra. But the decreased brightness in the darker part of the outer shadow region, the penumbra, is also apparent on the lunar disk. In fact, the relative size...

Explanation: Beautiful barred spiral galaxy M109, 109th entry in Charles Messier's famous catalog of bright Nebulae and Star Clusters, is found just below the Big Dipper's bowl in the northern constellation Ursa Major. In telescopic views, its striking central bar gives the galaxy the appearance of the Greek letter "theta", θ, a common mathematical symbol representing an angle. Of course M109 spans a very small angle in planet Earth's sky, about 7 arcminutes or 0.12 degrees. But that small angle corresponds to an enormous 120,000 light-year diameter at the galaxy's estimated 60 million light-year distance. The brightest member of the...

Explanation: What's that in the sky? It is a rarely seen form of lightning confirmed only about 25 years ago: a red sprite. Recent research has shown that following a powerful positive cloud-to-ground lightning strike, red sprites may start as 100-meter balls of ionized air that shoot down from about 80-km high at 10 percent the speed of light and are quickly followed by a group of upward streaking ionized balls. The above image, taken a few days ago above central South Dakota, USA, captured a bright red sprite, and is a candidate for the first color image ever recorded...

Explanation: How was the unusual Red Rectangle nebula created? At the nebula's center is an aging binary star system that surely powers the nebula but does not, as yet, explain its colors. The unusual shape of the Red Rectangle is likely due to a thick dust torus which pinches the otherwise spherical outflow into tip-touching cone shapes. Because we view the torus edge-on, the boundary edges of the cone shapes seem to form an X. The distinct rungs suggest the outflow occurs in fits and starts. The unusual colors of the nebula are less well understood, however, and speculation holds...

Explanation: Our Sun is not a giant blueberry. Our Sun can be made to appear similar to the diminutive fruit, however, by imaging it in a specific color of extreme violet light called CaK that is emitted by the very slight abundance of ionized Calcium in the Sun's atmosphere, and then false color-inverting the image. This solar depiction is actually scientifically illuminating as a level of the Sun's chromosphere appears quite prominent, showing a crackly textured surface, cool sunspots appearing distinctly bright, and surrounding hot active regions appearing distinctly dark. The Sun is currently near the maximum activity level in...

Explanation: What on Earth is that? The Richat Structure in the Sahara Desert of Mauritania is easily visible from space because it is nearly 50 kilometers across. Once thought to be an impact crater, the Richat Structure's flat middle and lack of shock-altered rock indicates otherwise. The possibility that the Richat Structure was formed by a volcanic eruption also seems improbable because of the lack of a dome of igneous or volcanic rock. Rather, the layered sedimentary rock of the Richat structure is now thought by many to have been caused by uplifted rock sculpted by erosion. The above image...

Explanation: Once the famous sunset comet, PanSTARRS (C/2011 L4) is now visible all night from much of the northern hemisphere, bound for the outer solar system as it climbs high above the ecliptic plane. Dimmer and fading, the comet's broad dust tail is still growing, though. This widefield telescopic image was taken against the starry background of the constellation Cepheus on May 15. It shows the comet has developed an extensive anti-tail, dust trailing along the comet's orbit (to the left of the coma), stretching more than 3 degrees across the frame. Since the comet is just over 1.6 astronomical...

Explanation: Above this boreal landscape, the arc of the Milky Way and shimmering aurorae flow through the night. Like an echo, below them lies Iceland's spectacular Godafoss, the Waterfall of the Gods. Shining just below the Milky Way, bright Jupiter is included in the panoramic nightscape recorded on March 9. Faint and diffuse, the Andromeda Galaxy (M31) appears immersed in the auroral glow. The digital stitch of four frames is a first place winner in the 2013 International Earth and Sky Photo Contest on Dark Skies Importance organized by The World at Night. An evocative record of the beauty of...

Explanation: Swinging around the Sun's eastern limb on Monday, a group of sunspots labeled active region AR1748 has produced the first four X-class solar flares of 2013 in less than 48 hours. In time sequence clockwise from the top left, flashes from the four were captured in extreme ultraviolet images from the Solar Dynamics Observatory. Ranked according to their peak brightness in X-rays, X-class flares are the most powerful class and are frequently accompanied by coronal mass ejections (CMEs), massive clouds of high energy plasma launched into space. But CMEs from the first three flares were not Earth-directed, while one...

Explanation: What happens when two galaxies collide? Although it may take over a billion years, such titanic clashes are quite common. Since galaxies are mostly empty space, no internal stars are likely to themselves collide. Rather the gravitation of each galaxy will distort or destroy the other galaxy, and the galaxies may eventually merge to form a single larger galaxy. Expansive gas and dust clouds collide and trigger waves of star formation that complete even during the interaction process. Pictured above is a computer simulation of two large spiral galaxies colliding, interspersed with real still images taken by the Hubble...

Explanation: What caused this mess? Some type of star exploded to create the unusually shaped nebula known as Kepler's supernova remnant, but which type? Light from the stellar explosion that created this energized cosmic cloud was first seen on planet Earth in October 1604, a mere four hundred years ago. The supernova produced a bright new star in early 17th century skies within the constellation Ophiuchus. It was studied by astronomer Johannes Kepler and his contemporaries, without the benefit of a telescope, as they searched for an explanation of the heavenly apparition. Armed with a modern understanding of stellar evolution,...

Explanation: It was just eight minutes after sunrise, last week, and already there were four things in front of the Sun. The largest and most notable was Earth's Moon, obscuring a big chunk of the Sun's lower limb as it moved across the solar disk, as viewed from Fremantle, Australia. This was expected as the image was taken during a partial solar eclipse -- an eclipse that left sunlight streaming around all sides of the Moon from some locations. Next, a band of clouds divided the Sun horizontally while showing interesting internal structure vertically. The third intervening body might be...

Explanation: Sometimes the sky above can become quite a show. In early September of 2010, for example, the Moon and Venus converged, creating quite a sight by itself for sky enthusiasts around the globe. From some locations, though, the sky was even more picturesque. In the above image taken in Spain, a crescent Moon and the planet Venus, on the far right, were captured during sunset posing against a deep blue sky. In the foreground, dark storm clouds loom across the image bottom, while a white anvil cloud shape appears above. Black specks dot the frame, caused by a flock...

Explanation: This week the shadow of the New Moon fell on planet Earth, crossing Queensland's Cape York in northern Australia ... for the second time in six months. On the morning of May 10, the Moon's apparent size was too small to completely cover the Sun though, revealing a "ring of fire" along the central path of the annular solar eclipse. Near mid-eclipse from Coen, Australia, a webcast team captured this telescopic snapshot of the annular phase. Taken with a hydrogen-alpha filter, the dramatic image finds the Moon's silhouette just within the solar disk, and the limb of the active...

Explanation: Face-on spiral galaxy M77 lies a mere 47 million light-years away toward the aquatic constellation Cetus. At that estimated distance, the gorgeous island universe is about 100 thousand light-years across. Also known as NGC 1068, its compact and very bright core is well studied by astronomers exploring the mysteries of supermassive black holes in active Seyfert galaxies. M77 is also seen at x-ray, ultraviolet, infrared, and radio wavelengths. But this sharp visible light image based on Hubble data follows its winding spiral arms traced by obscuring dust clouds and red-tinted star forming regions close in to the galaxy's luminous...

Explanation: As the New Moon continues this season's celestial shadow play, an annular solar eclipse track begins in western Australia at 22:30 UT on May 9 -- near sunrise on May 10 local time. Because the eclipse occurs within a few days of lunar apogee, the Moon's silhouette does not quite cover the Sun during mid-eclipse, momentarily creating a spectacular ring of fire. While a larger region witnesses a partial eclipse, the annular mid-eclipse phase is visible along a shadow track only about 200 kilometers wide but 13,000 kilometers long, extending across the central Pacific. For given locations along it,...

Explanation: To see a vista like this takes patience, hiking, and a camera. Patience was needed in searching out just the right place and waiting for just the right time. A short hike was needed to reach this rugged perch above a secluded cove in Julia Pfeiffer Burns State Park in California, USA. And a camera was needed for the long exposure required to bring out the faint light from stars and nebula in the background Milky Way galaxy. Moonlight and a brief artificial flash illuminated the hidden beach and inlet behind nearby trees in the above composite image taken...

Explanation: What caused the interestingly intricate tails that Comet Lemmon displayed earlier this year? First of all, just about every comet that nears the Sun displays two tails: a dust tail and an ion tail. Comet Lemmon's dust tail, visible above and around the comet nucleus in off-white, is produced by sun-light reflecting dust shed by the comet's heated nucleus. Flowing and more sculptured, however, is C/2012 F6 (Lemmon)'s blue ion tail, created by the solar wind pushing ions expelled by the nucleus away from the Sun. Also of note is the coma seen surrounding Comet Lemmon's nucleus, tinted green...